Solar Versus Superinsulation: A 30-Year-Old Debate

A dispute from the late ’70s and early ’80s still sheds light on energy-efficient design

Solar air collectors and a bin full of rocks. Sarah Sanford built this solar house on the cost of Maine in the late 1970s. Ductwork connects the vertical solar air collectors with an insulated rock bin under the floor slab. After the home was completed, Sanford realized that she should have installed insulation under her slab-on-grade foundation. The photo appeared in At Home In the Sun, a book published in 1979.

The oil price shock of 1973 sparked a burst of interest in “solar houses.” During the 1970s, owner-builders all over the U.S. erected homes with extensive south-facing glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. — sometimes sloped, sometimes vertical. Many of these houses included added thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night.
— concrete floors, concrete-block walls, or 55-gallon drums filled with water.

Some of these houses had passive solar features, while others included active hardware: space-heating systems that circulated water or antifreeze through roof-mounted collectors, or arrays of solar air collectors connected by ductwork to insulated rock bins in the basement.

Responding to a growing interest in all things solar, publishers came out with dozens of solar-house books in the late 1970s and early 1980s. It’s fun to re-read these old books — to see the photos of pony-tailed carpenters working on systems they claim will provide “free heat.” Once the warm glow of nostalgia fades, however, one begins to notice what’s missing from these books: any understanding of air leakage.

Most of these “solar houses” were built with little attention to air tightness and were insulated with fiberglass batts. Moreover, descriptions of homes with extensive ductwork never mentioned the need to seal duct seams.

Understanding air leakage

While these early solar houses were being built, researchers in several locations were making pioneering discoveries about how air leakage affects residential energy consumption:

In 1974, Princeton University’s Center for Energy and Environmental Studies received a federal grant to study residential heat loss. The center’s chief researchers were Ken Gadsby, Gautam Dutt, David Harrje, and Frank Sinden — a group later known as the “Princeton House Doctors.” In 1977, while investigating heat loss at townhouses in Twin Rivers, N.J., Gautam Dutt discovered previously undocumented air leakage paths through and around attic insulation. Dutt is credited as the discoverer of the “thermal bypass.”

Also in 1977, a group of Canadian researchers — including Robert Besant, Rob Dumont, David Eyre, and Harold Orr — built a model home in Regina, Saskatchewan to demonstrate the importance of airtight construction techniques. Dubbed the Saskatchewan Conservation House, the model home had an air leakage rate of only 0.8 ac/h @ 50 pascals.

In 1980, Ken Gadsby’s company, Gadsco, began selling the first commercially available blower doors[8].

The Saskatchewan Conservation House[6] marked the beginning of the superinsulation movement in North America. Inspired by the Canadian researchers’ emphasis on air sealing, thick insulation, and triple-glazed windows, a Massachusetts engineer named J. Ned Nisson organized a multi-city speaking tour for Harold Orr and Rob Dumont. The popularity of these workshops helped spread the word about superinsulation techniques throughout the U.S.

The following year, Gene Leger built a small superinsulated house with double-stud walls in Pepperell, Massachusetts. Leger’s house received widespread coverage in magazines and daily newspapers, and most of the articles mentioned that his annual heating bill was only $38.50. (For example, see John Ingersoll’s article, “Double-Wall House Minimizes Heat Loss and Fuel Bills,”[9] published in the October 1980 issue of Popular Science.)

Leger had designed a house without large areas of south-facing glazing, and his claim that airtightness and high levels of insulation were more important than solar gain were seen as revolutionary. By 1980, the groundwork had been laid for the great Solar versus Superinsulation debate.

Is it better to collect heat or minimize the need for heat?

Solar home designers of the 1970s wanted to maximize the collection of heat. According to the solar crowd, it was possible to design a solar heating system that could provide most of the space heating required for a single-family home — anywhere in the lower 48 states.

The superinsulation crowd had a different approach. Instead of maximizing the collection of heat, they designed houses that had minimal heating requirements. Superinsulation proponents noted, “It’s certainly possible to heat a home with the sun. But these solar houses are unnecessarily complicated, and many have comfort problems. They often overheat on sunny days and get too cold at night. If you build a superinsulated house without any solar features, your heating bills will be lower, your construction budget will be smaller, and you’re likely to be more comfortable.”

As a former editor of Energy Design Update (EDU), a superinsulation newsletter founded in 1982 by J. Ned Nisson, I tend to side with the superinsulation crowd. But I’m willing to listen to experts on the solar side of the debate.

Stories from a Solar Age editor

When EDU was founded, the rival periodical spearheading the solar side of the debate was Solar Age magazine. A few months ago, I asked Steve Bliss, one of the editors at Solar Age, to reminisce about his days at the premier solar magazine of the early 1980s.

“For one of my first articles for Solar Age, I interviewed two college professors living in a solar house in the Boston suburbs,” Bliss recalled. “When I got there, they were sitting in the house freezing — they were wearing down booties and down vests. They were suing their architect, who had used solar glazing formulas developed for houses in the Southwest.

“These passive solar houses just didn’t work in New England,” Bliss continued. “A lot of people were building solar houses that weren’t working. Companies started inventing things to fix these houses — solar shades, insulating shutters, rock bins. Solar Age analyzed all of these things, and we wrote that all of them failed. I wrote a lot of articles about what wasn’t working. There was a new perpetual motion machine every six months. Proprietary things, magic things. We used to call these houses ‘smart air houses’ — where the air would follow the blue arrows and the red arrows. The smart air knew where to go.

“What emerged over time was that the smart money was putting more effort on conservation,” Bliss said. “Passive solar is a weak heating system — so it’s really important to hang on to the heat you have. There was more and more interest in the building shell: Insulate really well and build a tight house. If you reduce the amount of glazing, you reduced a lot of your problems — overheating in the summer and excessive heat loss in the winter.”

Solar Age magazine ceased publication in 1986. Explaining the magazine’s decline, Bliss said, “At that time, interest in solar technology was dropping and interest in energy-efficient construction was growing.”

The lessons are still valid today

What lessons can we draw from the Solar Versus Superinsulation debate of the early 1980s?

Just because many solar houses of the 1970s and 1980s had design and construction flaws, doesn’t mean we should reject passive solar design principles or active solar heating systems.

South-facing glazing — especially sloped glazing — is a double-edged sword. While it admits plenty of heat — sometimes too much heat — on sunny days, it can lose a lot of heat on cold nights.

While active solar systems are expensive and require regular maintenance, thick insulation is long-lived and trouble-free.

The Achilles’ heel of many active solar systems is their parasitic energy use — that is, the electricity required to run pumps and blowers. Some solar air systems used so much electricity for fans that cynics concluded that most of the homes’ space heat came from waste heat emitted by the blower motors.

These days, the homes with the stingiest energy budgets — those complying with the Passivhaus standard[10] — don’t rely on active solar space heating systems. Although the orientation of windows is carefully considered, Passivhaus buildings don’t require large areas of south-facing glass.

Spending thousands of dollars on solar hardware for a house that hasn’t been carefully air-sealed and superinsulated is putting the cart before the horse. Nine times out of ten, if a designer takes the time to design a tight, well insulated envelope, the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. is so low that active solar equipment no longer makes sense.

If you really want an active solar heating system, go ahead and install one — but only if you’ve got a very tight, very well insulated house with triple-glazed windows.